Electrical resistor



Aug. 1, 1967 w. F. GLOVER ELECTRICAL RESISTOR Filed Oct. 13, 1965 Inventor WILL/AM A GLQVER By Attorney United States Patent 3,334,324 ELECTRICAL RESISTOR William F. Glover, Aldwych, London, England, assignor to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware Filed Oct. 13, 1965, Ser. No. 495,529 Claims priority, application Great Britain, Jan. 22, 1965, 2,887/65 Claims. (Cl. 338-216) ABSTRACT OF THE DISCLOSURE An electrical resistor is formed of a coaxial transmission line short circuited at one end. The outer conductor is a plurality of wires wrapped helically about the central conductor in contact therewith but insulated therefrom except at one end, the other end being connected to separate terminals. The line may be formed in a fiat spiral mounted on a flat support.

- This invention relates to electrical resistors and in particular to resistors for use at high frequencies.

Electrical resistors for use in high frequency alternating current circuits should offer the same resistance at all frequencies and should therefore be as non-reactive as possible.

Several ways are known to reduce the residual reactances of a resistor. For example, in composition high stability carbon or metal film resistors, the resistive ele ment is either made of a body of compressed granular substance of high specific resistance or of a thin film of carbon or metal deposited on an insulating former.

For measuring purposes and in other applications requiring a high order of reliability and long term stability, wire wound resistors are preferred. These resistors tends to have higher residual reactances and special winding arrangements are known to reduce this effect.

When it is more important to be able to calculate the magnitude of the spurious reactances rather than reduce them to a minimum, resistors of simple geometrical shape, for example in the form of a rigid coaxial transmission line are used. These resistors tend however to be of large size.

To overcome this disadvantage there is provided according to the invention an electrical resistor comprising a length of flexible transmission line short circuited at one end, said line including a central conductor made of resistive material, not less than five screening wires wrapped helically round the central conductor to form a screen, the central conductor and said screen being insulated from one another along the entire length; but electrically connected to each other at one end, the other end of the central conductor and of the screen being connected to termials, said termials and the coiled up flexible transmission line being attached to a support or former.

The invention will now be described with reference to the accompanying drawings in which:

FIG. 1 shows a sectional view of the flexible transmission line and FIGS. 2 and 3 show a plan and a sectional view, respectively, of a resistor assembly according to one embodiment of the invention.

A sectional view of the composite conductor constituting the resistive transmission line of the resistor is shown in FIG. 1. The line comprises a central conductor 1 for example a length of resistance wire. This resistance wire is helically wrapped with at least five screening conductors 2 made of copper or other material having a relatively high electrical conductivity. The central conductor is insulated from the screening conductors by providing either or both sets of conductors with suitable electrical insulation, as indicated by 3 for example enamel or silk.

The diameter of the central conductor is selected to give the required resistance per unit length, and have the required current carrying capacity. The diameter of the screening wires is chosen to surround the periphery of the central conductor without leaving any significant gaps between the conductors.

At one end the central conductor is connected electrically to the conductors of the screen. In this way a length of a coaxial transmission line shorted at one end is realised.

Details on the performance of short circuited resistive transmission lines are given in an article, Radio Frequency Resistors as Uniform Transmission Lines, by D. R. Crosby and C. H. Pennypacker in Proceedings of the I.R.E., February 1946.

The length of flexible, short circuited transmission line used in the resistor depends on the required value of resistance and on the diameter of the central conductor. It is however important that if the residual reactances of the resistor are to be a minimum, over a wide range of frequencies the electrical length of the transmission line should not exceed say 10% of the wavelength of signal carried by the resistor and the diameter of the central conductor should be kept as small as possible. Thus if a resistor is required to have a value necessitating a line in excess of a certain length, it is better to use several low value resistors in series rather than a single high value resistor. Similarly if a relatively low value resistor is required it is better to use a number of higher valued resistors connected in parallel than a single low value resistor having a thicker central conductor.

For convenience of handling and mounting the flexible transmission line is coiled, provided with terminals and attached to a suitable support or former. A preferred mounting ararngement is shown in FIGS. 2 and 3.

The mounting base 4 is a circular plate of ceramic material to which by means of cement are secured two terminals 5 and 6. The transmission line resistance element 7 is coiled into a flat spiral and is secured to one flat face of the base 4 by suitable adhesive or cement. The short circuited end 8 of the transmission line lies on the outside of the spiral. The open circuited end of the line is pulled through hole 9 in the base. The central conductor 1 of the line is soldered to one of the terminals while the conductors forming the screen are soldered to the other terminal.

If as, stated above, the resistor comprises several transmission lines connected either in series or in parallel the lines are coiled together to form a multi-start flat spiral having their short circuited ends on the outside of the spiral and their free ends in the centre where they are interconnected as required and taken to the terminals.

In order to protect the resistor from the effects of humidity and to give mechanical protection a coat 10 of epoxy resin is provided.

It is to be understood that the foregoing description of specific examples of this invention is not to be considered as a limitation on its scope.

What we claim is:

1. An electrical resistor comprising a length of flexible coaxial transrnision line short circuited at one end, said line including a central conductor made of resistive material, a plurality of separate screening wires wrapped helically round the central conductor to form an outer screen, 'the central conductor and said screen being insulated from and in contact with one another along the entire length, but electrically connected to each other at said one end, the other end of the central conductor and of the screen being connected, respectively, to separate terminals insulated from one another, said terminals and transmission line being attached to a support.

2. A resistor as claimed in claim 1 in which the support is made of insulating material having a hole therethrough, said insulating material having the shape of a flat disc to one flat face of which one end of said transmission. line is attached, the other end extending through said hole and the opposite flat face is provided with two terminals which are connected respectively to the central conductor and to the screen at said other end of the transmission line.

3. A resistor as claimed in claim 2 in which said discshaped support is provided with a central aperture through which said other end of the transmission line is taken to the terminals.

4. The device of claim 1 wherein said plurality of screening wires comprises at least five wires.

5. The device of claim 2 wherein the length of said central conductor and screen are wound in a fiat spiral mounted on said fiat face, the outside end of the spiral having inner and screen conductors connected together and the inside end having the inner conductor and screen connected respectively to said two terminals.

References Cited UNITED STATES PATENTS 2,187,213 1/1940 Milliken 174l14 2,294,881 9/1942 Alford 338216 2,434,560 1/1948 Gunter 338216 2,446,387 8/1948 Peterson 174-105 2,771,565 11/1956 Bryant et a1. 338216 3,176,248 3/1965 McHenry 333-22 RICHARD M. WOOD, Primary Examiner.

W. D. BROOKS, Assistant Examiner. 

1. AN ELECTRICAL RESISTOR COMPRISING A LENGTH OF FLEXIBLE COAXIAL TRANSMISSION LINE SHORT CIRCUITED AT ONE END, SAID LINE INCLUDING A CENTRAL CONDUCTOR MADE OF RESISTIVE MATERIAL, A PLURALITY OF SEPARATE SCREENING WIRES WRAPPED HELICALLY ROUND THE CENTRAL CONDUCTOR TO FORM AN OUTER SCREEN, THE CENTRAL CONDUCTOR AND SAID SCREEN BEING INSULATED FROM AND IN CONTACT WITH ONE ANOTHER ALONG THE ENTIRE LENGTH, BUT ELECTRICALLY CONNECTED TO EACH OTHER AT SAID ONE END, THE OTHER END OF THE CENTRAL CONDUCTOR AND OF THE SCREEN BEING CONNECTED, RESPECTIVELY, TO SEPARATE TERMINALS INSULATED FROM ONE ANOTHER, SAID TERMINALS AND TRANSMISSION LINE BEING ATTACHED TO SUPPORT. 